The valve-regulated lead-acid battery has been developed as an advanced lead-acid battery. A valve-regulated lead-acid battery filled with gel electrolyte, a so-called gel battery, is often used for industrial applications at deep discharge and deep charge cycle operation as its immobilized gel electrolyte reduces acid stratification and eventually extends the battery service life. As another type of valve-regulated lead-acid battery, using absorbed glass mat separators, so called AGM battery is often used for automotive applications.
Conventionally, small amount of carbon black has been added to negative electrode active material of lead-acid battery. Generally, about 0.2 wt % of carbon black is added in the negative paste regardless whether the battery is valve-regulated or conventional flooded type. This amount is thought to be sufficient enough to improve the formation process.
Recently, high loading of conductive carbon materials such as carbon black, graphite has been investigated to attempt to improve performances of valve-regulated lead-acid battery, especially with an absorbed glass matt separator design.
U.S. Pat. No. 5,547,783 discloses adding 0.5 to 7.5 wt % of conductive additives such as carbon black, acetylene black, polyaniline, tin powder, tin compound powder, etc. to a negative active material of a valve-regulated lead-acid battery with absorbed glass matt separator, where the theoretical capacity of negative active material in the battery is less than that of the positive active material. This variation led to an extended battery cycle life.
US 2009/0325068 discloses that the concentration of up to 6% of graphite or mixtures of carbon black and graphite minimized the accumulation of lead sulfate on the surface of the negative plate during high rate partial state of charge battery operation and enhanced battery performance.
There are several drawbacks regarding usage of conductive carbon additives to lead acid battery. Mostly, improved battery performances at the partial state of charge operation of VRLA battery with absorbed glass matt separator have been reported.
For VRLA battery filled with gel electrolyte, especially, in a deep charge and discharge cycle operation, not many investigations are reported. Adding higher amount of low density material such as carbon black or graphite to the negative paste typically requires adding more water in order to obtain suitable processability, but this in turn results in certain disadvantages, such as, a reduced paste density and a very poor active mass adhesion.
Japanese patent number 43-64460 teaches that the combination of 1 to 5 wt % of a carbon material having a high specific surface area and 1 to 5 wt % of conductive carbon material in a negative active mass of valve regulated lead-acid battery with absorbed glass matt separator gave an advantage to high rate discharge performance at the partial state of charge battery operation.
US 2010/0015531 discloses adding 1 to 2 wt % of a carbon material having a high specific surface area, e.g., having a meso-pore volume of greater than about 0.1 cm3/g. The resultant battery maintains automotive battery performances despite containing less lead.
Several drawbacks regarding the use of carbon material having a high specific surface area for negative active material of lead-acid battery, particularly a valve regulated lead-acid battery with an absorbed glass mat separator or automotive application are mostly reported. For a valve regulated lead-acid battery with a gel electrolyte, especially in a deep discharge and charge cycle operation, fewer drawbacks are disclosed.
However, it remains a technical challenge to improve deep cycling performance of the gel electrolyte filled valve regulated lead acid batteries while maintaining discharge performance. The inventors have sought to overcome this technical challenge by the compositions disclosed herein.